The most commonly used and available strings are 100% cotton. These strings are easy on the fingers and work very well with the common soft wood axles. Rarely, you may come across strings using artificial fiber ("hard strings"); ditch them and find a reliable supplier of cotton strings. One very nice characteristic of cotton strings is that they are "stretchy", which reduces the shock when you throw a fast sleeper (see the previous article "Restful Sleepers").
Strings are inexpensive, so keep a generous supply on hand. Then make it a practice to replace your YoYo string often; certainly before it becomes noticeably dirty and absolutely before it breaks!
When you mount a string on your YoYo, here's a procedure that will help you get best YoYo performance:
This procedure assures you that there are no kinks in the string and no uneven winds anywhere near the YoYo and certainly none in the YoYo groove.
The YoYo string always needs to be retired before it dies. When it looks dull and grey, when it has developed a noticeably and permanent tight twist, or when it has lost its supple and stretchy feel - replace it! Any of these conditions will downgrade the performance of your YoYo. Many YoYo players use strings far beyond their effective and useful life. Don't make that mistake.
Remember, "The String's the Thing" that gets you to the State of YO!